An increasing volume of communications traffic is being transported over Internet Protocol (IP) networks. In a typical arrangement, user terminals connected to respective local networks (PSTN) are provided with communications links over an IP network via media gateways which form an interface between each PSTN and an IP network. The media gateways handle all the bearer traffic switching, media adaptation (Time Division Multiplex and Internet Protocol conversion) and voice services (compression, echo cancellation, announcements, etc.). The gateways are controlled by a media gateway controller which handles all the call signalling (e.g. SS7, H.248, etc) and control functionality.
Calls between terminals will either be voice calls, fax calls or modem calls. When a call is being set up, the system does not ‘know’ what type of call is to be established, and therefore an assumption is made that a voice call is intended. Once a path over the IP network between the media gateways has been established, the call can commence, at which point it becomes apparent whether a voice, fax or modem connection is required. If a fax or modem call is intended, the calling terminal sends a tone characteristic of that type of call. This tone is recognised by the media gateways which, in response, send messages to the media gateway controller. The media gateway controller then instructs the media gateways to transition to the appropriate type of call. Thus, intervention by the media gateway controller is required both at the initial call set up and for any subsequent transition from the voice call mode e.g. to the fax mode. This of course requires message exchanges between the media gateway controller and the media gateways leading to congestion and delay under busy conditions.
A particular problem can arise between manual single page (G3FE) fax terminals in which, during a facsimile connection, the terminals go to voice and then back to fax. In such cases, the media gateways inform the media gateway controller of every tone and let the media gateway controller decide when to transition from voice to fax (for the case of H.248) or renegotiate support of fax between the media gateways (as is the case of H.323 annex D or SIP MGs as described in Rec. T.38 annex D). If this communication is carried over networks that are lossy and/or have considerable delay, the fax terminals may time out thus preventing the set up of the FoIP call.
Further, every time a media gateway sends a tone notification message (e.g. indicating that a CNG, CED or V21 preamble was detected) to the H.248 media gateway controller, the media gateway controller responds with an ACK message and, maybe, a new message (i.e. a Modify message) in which the T.38 capabilities are renegotiated between the media gateway via the media gateway controller. It any of these messages are lost, corrupted or heavily delayed, the message is resent after a set time (which cannot be to short, to avoid unnecessary messages being sent due previous messages being delayed), which together with the normal delay of an IP network can exceed the fax terminal G3FE timers resulting in the T.38 fax call being aborted. The delay between T.38 transitioning related messages is increased if the messages have to be exchanged between different media gateway controllers that may support different call control protocols and be located physically at a distance and connected by an IP network.